If wireless communication coverage is relatively good, both an operator and a government wish that some public safety events could be handled by wireless communication. After a safety event occurs, an emergency commanding team and a rescue team perform communication in a geographic area. Each team member holds a special terminal. This special terminal is referred to as a device-to-device (D2D) communications device or a cellular D2D communications terminal device. That is, like an ordinary wireless terminal, this terminal can perform cellular communication with a base station, and can also directly communicate with another cellular D2D communications terminal device. Due to blocking of a terrain and a time-varying feature of radio signals, there is actually a cell coverage hole. In public safety requirements, it is required that a cellular D2D communications terminal can be applied within network coverage and without network coverage. Generally, public safety personnel are multiple persons that are grouped or clustered to take actions.
In conventional long term evolution (LTE) system, an asynchronous hybrid automatic repeat request (HARQ) mode is used for transmitting downlink data, and a synchronous HARQ mode is used for transmitting uplink data.
However, if an LTE HARQ mode is used in D2D communication, because there is no feedback channel between transmitting user equipment (UE) and receiving UE, the LTE retransmission mode cannot be used to ensure reliability of data transmission.